THE chj<;tognatha, or peimitive mollusca. 371 



and the great cerebro-visceral connectives which lead back 

 to the ^'ventral" or visceral ganglion. Lateral extensions of 

 the cerebral ganglion give off two nerves, which are, at any 

 rate, partly motor, to the cephalic or buccal muscles, and a 

 pair of lateral nerves to the integument of the head. 



To demonstrate the existence of ventral commissures below 

 the oesophagus is not alwaj^s an easy matter. Langerhans 

 definitely affirmed the presence of a circumoesophageal ring 

 completed by a subcutaneous commissural nerve just behind 

 the mouth, a statement which Hertwig was not able to 

 confirm. My own observations on Neapolitan material have 

 led me to believe tbat there is a commissural plexus of 

 ganglion cells beneath the skin, which, if more concentrated, 

 might give rise to a commissural nerve strand such as 

 Langerhans figures. Such a nerve-loop is to be compared 

 with the stomatogastric loop of the Mollusca, the commissural 

 nerves of which are often extremely attenuated, and, owing 

 to their position on the buccal muscles, difficult to demon- 

 strate; in Nautilus, for example, the completion of the 

 buccal nerve-loop has only been recently proved by Graham 

 Kerr. The co-existence of two pairs of ganglia, which may 

 be termed buccal and pharyngeal in Chastoguatha, in many 

 Gastropoda (e. g. Patella), and in Nautilus is a significant 

 feature. 



The descriptions of Hertwig and of Gourret differ in 

 regard to the mode of origin of certain nerves from the 

 cerebral mass, but they may be reconciled if we suppose the 

 cerebro-buccal connectives and motor nerves to the muscles 

 of the mandibleSj to run side by side for a short distance 

 after leaving the cerebral ganglion in Sagitta hexaptera 

 (Hertwig), but to be separate from the start in Spadella 

 marioni (Gourret). 



If it could be established that the buccal ganglia of the 

 Chastognatha are derived from the stomodasal ectoderm the 

 resemblance to the Mollusca would be complete. 



Concerning the existence of the posterior or visceral loop 

 there can be no doubt. The visceral ganglia appear early as 



